1. Field of the Invention
The invention relates to a sensor for noncontact scanning of a surface with an oscillating beam made from a piezoelectric material, wherein the beam is designed for a transverse vibration of a free end of the beam, with an electrically conductive tip extending in transverse direction at the free end of the beam, wherein the beam is provided with a first deflection electrode and a second deflection electrode, which second deflection electrode is inversely polarized with regard to the first deflection electrode in order to collect charges that are generated when the beam is deflected. Such a sensor is, in particular, suitable for combined atomic force microscopy that is a combination of atomic force microscopy with other scanning probe microscopies. One example is combined scanning tunneling and atomic force microscopy.
2. Description of Related Art
Scanning probe microscopy works by scanning a sharp tip over a surface (in x- and y-direction) while keeping the interaction force between tip and sample constant by means of a feedback circuit that controls the tip height z such that an image z(x,y) is formed. Image contrast is defined by the tip-sample interaction. Two basic methods are distinguished: imaging with repulsive and imaging with attractive interactions. When the tip approaches the sample, the force is initially attractive. Once tip and sample “touch” each other, the force becomes repulsive. Scanning tunneling microscopy relies on a similar principle, but it requires electrically conductive tips and samples. Instead of the force, a current is measured (from about 100 fA to about 1000 nA), that flows once a voltage bias is applied and the distance between tip and sample is sufficiently small (between 0.2 nm and 2 nm).
The force is measured by mounting the probe tip onto a cantilever spring. In quasi-static force microscopy, the static spring deflection is measured. In dynamic force microscopy, the cantilever oscillates and an observable, such as the oscillation amplitude, the frequency or the oscillation phase with respect to a sinusoidal drive signal are measured. In principle, it is possible to simultaneously measure the force (or a quantity derived from it such as the force gradient), by measuring the deflection of the cantilever or a quantity derived from the deflection as well as the tunneling current that flows between tip and sample.
FIG. 1 is a schematic view of a known sensor for simultaneous tunneling- and force microscopy. The sensor (stiffness k) oscillates at amplitude A. The unperturbed resonance frequency is f0=(k/m*)0.5/2π, and the frequency changes to f=((k+<kts>)/m*)0.5/2π. The influence of a tip-sample force gradient <kts> leads to a frequency shiftΔf=<kts>/(2k)f0,  (G1. 1)where m* is the effective mass of the cantilever. The frequency shift can be used as a feedback signal to control the distance of a probe tip that scans the sample. When both tip and sample are electrically conductive and a bias voltage is applied, a tunneling current flows that is modulated by the oscillation and contains important information about the sample. With metallic tips and samples, the tunneling resistance of the vacuum gap is approximately given by 12.9 kΩ×exp(−2 z/100 pm). A simultaneous measurement of force and current is desirable, because that extends the applicability of scanning probe microscopy.
Piezoelectric sensors, such as the qPlus Sensor (see, e.g., German Patent DE 196 33 546 C2 and F. J. Giessibl, Applied Physics Letters 73, 3956, 1998 and F. J. Giessibl, Applied Physics Letters 76, 1470, 2000) and the so-called needle sensor (see, e.g. K. Bartzke et al., International Journal of Optoelectronics 8, Nos. 5/6, 669-676, 1993; T. An et al. Appl. Phys. Lett. 87, 133114, 2005) are formed of a quartz beam, whose lateral deflection (qPlus) or length extension (needle sensor) is measured by means of the piezoelectric effect. The quartz beam is covered by two pairs of electrodes (qPlus) or two single electrodes (needle sensor), that collect charges for a constant deflection and generate an alternating current when oscillating.
The qPlus sensor 10 shown schematically in FIG. 2 utilizes the beam deflection, where a deflectable beam 12 with a tip 14 is mounted to a base part in rest. The piezoelectric effect transforms mechanical strain caused by deflection to surface charges that are collected by the electrodes that cover the quartz beam. When the beam is deflected upwards as shown in FIG. 2, the upper half of the beam is subject to tensile strain, the lower half to compressive strain.
The emergence of a surface charge density σel with the presence of a mechanical stress σmech is due to the piezoelectric effect, where the surface charge density is given by:σel=d12σmech.  (eq. 1)
The prefactor d12 is the piezoelectric coupling constant with a typical value of about 2.3 pC/m for quartz. The mechanical strain leads to surface charges, that are collected by electrodes and transferred to an amplifier. The geometric arrangement of the electrodes is chosen such that a maximal charge is delivered for a given bending symmetry in order to obtain a maximal signal-to-noise ratio in deflection measurement. The basically sinusoidal deflection transforms into a basically sinusoidal alternating current.
When intending to measure the tunneling current in parallel, the electrically conductive tip needs to be connected to the outside. Prior art (F. J. Giessibl, Applied Physics Letters 76, 1470, 2000) utilizes one electrode of the quartz beam to guide the tunneling bias voltage to the tip. The tunneling current is collected at the sample. One disadvantage of this arrangement is that the tunneling current needs to be collected at the sample and deflection signal and a galvanic separation between deflection signal and tunneling current is not feasible. The sample needs to be electrically isolated from the body of the scanning probe microscope. This is a clear disadvantage, in particular in low-temperature microscopes, because the sample should be thermally connected well to the cooling bath to allow for low sample temperatures. As stated by the Wiedemann-Franz law, a good electrical connection ensures a good thermal connection and vice versa. In addition, the sample and the sample holder are generally much larger than the tunneling tip, therefore they have a larger electrical capacity with respect to ground (typically tens of pico-Farads). Large capacity to ground has the disadvantage of limiting the bandwidth of the tunneling current measurement and increasing its noise figure.
German Patent Application DE 195 13 529 A1 relates to a needle sensor, that contains a drive electrode on opposite faces of the beam to drive the beam into resonant vibration. One of the faces contains an additional electrode that connects to the tip to enable tunneling current measurement.